(function (doc,win) {
    var docEl = doc.documentElement,
    resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    recalc = function () {
        var clientWidth = docEl.clientWidth;
        if (!clientWidth)return;
        docEl.style.fontSize = 20 * (clientWidth/320) + 'px'
    };
    if(!doc.addEventListener) return;
    win.addEventListener(resizeEvt, recalc, false);
    doc.addEventListener('DOMContentLoaded', recalc, false);

})(document,window);